Diesel-fueled internal combustion engines are used to power vehicles such as medium and heavy duty trucks. Diesel engines are also used in stationary power generation systems. While exhaust aftertreatment systems for gasoline engine have been widely used since the 1970s, diesel engine aftertreatment systems have only recently come into widespread use.
Whereas gasoline engines use spark ignition, diesel engines use compression ignition. As a consequence, the composition of diesel exhaust is much different from that of gasoline engines. The major pollutants in gasoline engine exhaust are carbon monoxide, unburned hydrocarbons, and some NOX. The major pollutants in diesel engine exhaust are NOX and particulate matter (soot).
A catalytic converter, which is an exhaust aftertreatment device comprising a so-called three-way catalyst, can effectively control gasoline engine emissions by oxidizing carbon monoxide and unburned hydrocarbons while also reducing NOX. This approach is unsuitable for diesel engine exhaust because diesel exhaust contains from about 4 to 20% oxygen. The excess oxygen and dearth of oxygen accepting species (reductants) makes catalytic converters ineffective for reducing NOX in diesel exhaust.
Several solutions have been proposed for controlling NOX emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOX emissions. Another set of approaches remove NOX from the vehicle exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR) catalysts, and lean NOX traps (LNTs).
Lean-burn NOX catalysts promote the reduction of NOX under oxygen-rich conditions. Reduction of NOX in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOX catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOX reduction, adding 3% or more to the engine's fuel requirement. Currently, the sustainable NOX conversion efficiencies provided by lean-burn NOX catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOX by ammonia. The reaction takes place even in an oxidizing environment. The NH3 can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOX reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOX under lean conditions and reduce and release the adsorbed NOX under rich conditions. An LNT generally includes a NOX adsorbent and a catalyst. The adsorbent is typically an alkali or alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust (exhaust containing an excess of oxygen and other oxidizing species in comparison to reducing compounds), the catalyst speeds reactions that lead to NOX adsorption. In a rich exhaust (containing reductants in excess of oxidizing compounds), the catalyst speeds reactions by which reductants are consumed and adsorbed NOX is reduced and desorbed. In a typical operating protocol, deNOX operations, which include producing a rich environment within the exhaust, are carried out from time-to-time to regenerate (denitrate) the LNT.
In addition to accumulating NOX, LNTs accumulate SOX. SOX is the product of combusting sulfur-containing fuels. Even with low sulfur diesel fuels, the amount of SOX produced by combustion is significant. SOX adsorbs more strongly than NOX and necessitates a more stringent, though less frequent, regeneration (desulfation or deSOX operation). A deSOX operation requires more time than a deNOX operation. Desulfation requires elevated temperatures, e.g., 700° C. Accordingly, a deSOX operation generally comprises a heating phase followed by a rich phase in which the SOX is actually removed. Ideally, rich conditions are maintained continuously until the deSOX operation is complete, but it may be necessary to periodically interrupt the rich phase to avoid overheating. Whereas a deNOX operation can be completed in a few seconds, deSOX operations take several minutes, commonly on the order of 5-15 minutes.
Using an LNT to control NOX emissions from a diesel-powered vehicle causes a fuel penalty of several percent, the fuel penalty being the increase in the vehicle's fuel consumption in comparison to running the vehicle without exhaust aftertreatment. Most of the fuel penalty is the cost of carrying out deNOX operations. All other factors being equal, reducing the frequency of deNOX operations to the minimum required reduces the fuel penalty. However, the fuel penalty is dependent on the conditions prevailing during deNOX operations, particularly LNT temperature, exhaust oxygen concentration, and exhaust flow rate. Fuel can be saved by preferentially carrying out deNOX operations when conditions are more suitable. Minimizing the fuel penalty as a whole involves balancing the urgency of the need to regenerate with the suitability of present (or anticipated) conditions for carrying out a deNOX operation.
U.S. Pat. No. 6,615,579 describes an exhaust purification system in which deNOX operations are carried out when the NOX loading of an LNT reaches a threshold level. The deNOX operations comprise running the engine rich. The threshold is set higher when the engine load is high in order to reduce fuel consumption and improve engine durability.
U.S. Pat. No. 6,460,329 describes another method in which deNOX operations are carried out when an LNT reaches a threshold loading level. The threshold is adapted to account for aging. Aging is assessed from LNT performance immediately following each regeneration.
In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel requirement), and reduces NOX emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations that limit NOX emissions from trucks and other diesel-powered vehicles.